1. Field of the Invention
The present invention relates to a numerical controller of a machine tool and, more particularly, to a numerical controller of a machine tool that performs drilling.
2. Description of the Related Art
For drilling a hole in a workpiece, a drilling apparatus is known that causes a relative rotary motion and a relative feeding motion between a workpiece and a tool, and thereby drills a hole. In conventional drilling, predetermined fixed speeds were used as the feed speed for the feeding motion and the rotation speed for the rotary motion. This may cause a problem in that the tool receives a large load when contacting the workpiece, possibly affecting the service life and the straightness of the tool.
As prior art techniques concerning the speed control of tools in drilling, the techniques disclosed in the patent documents cited below are known.
Japanese Patent Application Laid-Open JP11-216640A discloses a technique described below. In a process of cutting, segments of the machining area in a workpiece are defined as a chamfer, a depth, a penetration in the order of machining for each of which a cutting operation for a fixed depth of cut is repeated. The cutting feed speed of the tool is set to be higher in the order of at the depth, at the chamfer, and at the penetration and the rotation speed of the tool is set to be lower at the penetration than at the chamfer and at the depth. The feed speed and the spindle rotation speed are thus changed to predetermined speeds for each of the defined areas when machining of the area is started.
Japanese Patent Application Laid-Open JP5-050311A discloses a technique described below. In deep hole machining, the feed speed of the cutting shaft is temporarily decreased to the stress-removing feed speed at predetermined intervals, whereby the stress applied on the cutting tool is reduced or relieved to become zero. While this operation is performed, the bent shape of the cutting tool is restored to the original state thereof with the elasticity thereof.
Japanese Patent Application Laid-Open JP2002-120219A discloses a technique described below. In a method of piercing a hole into a fragile material, after the tip of the tool reaches a specific point that is a specified distance away from the piecing point inside the fragile material until the tip of the tool passes through the piercing point, the tool is advanced with the speed thereof switched to a low speed, whereby cracks are prevented on the back face of the fragile material.
Japanese Patent Application Laid-Open JP11-065633A discloses a technique of changing the ratios of the feed speed and the spindle rotation speed with respect to a specified area using an override function.
Japanese Patent Application Laid-Open JP2004-141991A discloses a technique described below. In tap hole forming work for forming a hole in a workpiece having a prepared hole, a tap tool that is rotated by a spindle is inserted in and removed from the workpiece to form a hole, of which the depth and the diameter are predetermined. A torque required for cutting is calculated and the rotational angular acceleration is adequately controlled, whereby the torque of the spindle motor is sufficiently utilized and the work efficiency is improved.
Japanese Patent Application Laid-Open JP2012-016793A discloses a technique described below. When a hole is formed in a workpiece having a plurality of hole-forming areas made of different materials and thus segmented, the load torque applied on the tool is used for determining change of materials and the feed speed and the spindle speed are automatically controlled for each different material.
The technique disclosed in Japanese Patent Application Laid-Open JP11-216640A is a technique of changing the feed speed and the spindle rotation speed according to the chamfer, the depth, and the penetration. However, the technique changes the feed speed and the spindle rotation speed after the tool has come in contact with the workpiece. This may cause a problem in that the specified speeds have been reached before the start of a cutting-in operation and adequate acceleration thus cannot be performed during cutting.
The technique disclosed in Japanese Patent Application Laid-Open JP5-050311A decreases the feed speed at predetermined intervals. However, the purpose of the decreasing is to reduce or zero out the stress applied on the tool to restore the bent shape of the cutting tool. Furthermore, because the technique does not control the spindle rotation speed, the load applied on the tool may not be sufficiently reduced.
The technique disclosed in Japanese Patent Application Laid-Open JP2002-120219A switches the feed speed of the tool to a low speed at a specific point inside a fragile material. However, the technique switches the feed speed at a specific point inside a fragile material which is the workpiece. This may cause a problem in that the specified speeds have been reached before the start of a cutting-in operation and adequate acceleration thus cannot be performed during cutting. Furthermore, because the technique does not control the spindle rotation speed, the load applied on the tool may not be sufficiently reduced.
The technique disclosed in Japanese Patent Application Laid-Open JP11-065633A specifies areas inside the workpiece and controls the feed speed and the spindle rotation speed for each area. However, the technique controls the feed speed and the spindle rotation speed after the tool come in contact with the workpiece, and it is thus difficult to reduce the load applied on the tool contacting the workpiece during drilling.
The technique disclosed in Japanese Patent Application Laid-Open JP2004-141991A is intended for forming a tap hole in a workpiece having a prepared hole and based on the assumption that the prepared hole has been formed before machining. The purpose of the machining is forming a tap hole, that is, forming a screw groove in the hole, and it is thus difficult to reduce the load applied on the tool contacting the workpiece during drilling.
The technique disclosed in Japanese Patent Application Laid-Open JP2012-016793A detects the load torque applied on the tool coming in contact with the workpiece to change the machining speed. However, the technique changes the feed speed and the spindle rotation speed after the tool has come in contact with the workpiece, and it is thus difficult to reduce the load applied on the tool contacting the workpiece during drilling.